Because of its ready availability, low cost and potential for reducing particulate emissions, natural gas is a promising substitute for diesel fuel for fuelling compression ignition engines, which are commonly known as “diesel-cycle” engines. While natural gas is disclosed as an example of a preferred fuel for the disclosed apparatus and method, persons familiar with the technology involved here will understand that these developments can also be implemented with other suitable fuels, such as hydrogen, propane and any other fuels that can be substituted for diesel fuel for fuelling an engine, while substantially matching the performance of a conventional diesel-fuelled engine, and/or lowering emissions of particulate matter and/or oxides of nitrogen (“NOx”).
Conventional methods of introducing gaseous fuel into an engine premix all the gaseous fuel with the intake air, which is a method known as “fumigation”. Engines using such a method have been unable to match the power, performance, and efficiency of diesel-fuelled engines, mostly because the amount of fuel that can be introduced in this manner can be limited in certain operating conditions because of the risk of engine knock, which happens when there is premature detonation fuel. It has been found that the favorable operating characteristics and high efficiency of conventional diesel-fuelled engines can be matched when at least some of the gaseous fuel is introduced directly into the engine's combustion chamber, late in the compression stroke. Accordingly, there is a need for an apparatus and method for injecting gaseous fuel directly into the combustion chamber.
A challenge with substituting gaseous fuels such as natural gas for liquid fuel in an engine designed to be fuelled with liquid fuels such as diesel fuel, is that, compared to liquid fuels like diesel fuel, much higher temperatures and pressures are typically needed to auto-ignite gaseous fuels. Therefore, in order to burn a gaseous fuel in a conventional engine with the same compression ratio without having to completely redesign the engine, some mechanism is required to assist with ignition of the gaseous fuel, such as a hot surface provided by a glow plug, or a fuel injection valve for introducing a pilot fuel. The pilot fuel can be a small quantity of liquid fuel, such as diesel fuel, which auto-ignites to trigger the ignition of the gaseous fuel.
Using a pilot fuel in addition to a main charge of gaseous fuel can require the installation of two independently operable injection valves, one for the pilot fuel and one for the gaseous main fuel. To avoid having to redesign and replace the cylinder head to accommodate two injection valves, it is preferable to employ one single valve for the injection of both the liquid and gaseous fuels that fits into the same opening as a conventional diesel fuel injection valve. The applicant's co-owned U.S. Pat. Nos. 6,073,862, 6,336,598, 6,439,192, 6,761,325 and 7,124,959, describe different embodiments of a dual fuel injection valve comprising a dual needle assembly. These patents describe preferred embodiments in which the outer and inner needles are each movable independently from each other between respective open and closed positions to independently inject the gaseous and the liquid fuels directly into the combustion chamber of an internal combustion engine. While using such a dual fuel injection valve facilitates installation in cylinder heads designed to receive a single diesel fuel injection valve, the construction of a fuel injection valve with two independently operable needles is somewhat complex and its assembly requires more precision and care compared to conventional single fuel injection valves that only have one needle.
Therefore it can be advantageous to use an injection valve having only one needle movable between a closed and an open position that injects both the gaseous and the liquid fuels into the combustion chamber of an engine when the valve is in its open position. Such an injection valve is described in published PCT/International Publication No. WO 2008/000095. Liquid fuel is passed through a bore provided within the needle and introduced into a chamber of the injection valve where it is entrained in the gaseous fuel during an injection event. The liquid fuel is atomized by the flow of gas to the combustion chamber. While the overall construction of the injection valve is simplified compared to a dual needle solution, the construction of a needle with an internal bore is still complex.
While there has been some progress in simplifying the construction of the injection valve for injecting a gaseous fuel and a liquid fuel into the combustion chamber of an internal combustion engine, there is still a need to further reduce the complexity of the injection valve construction.